I'm not sure about Cornwell, but with both Rice and Hamilton, once both of those writers achieved enough success they did away with both proofreaders and editing. With Hamilton in particular, the work reads like a really bad first draft. I mean a
really bad first draft. But the publishers let them (and others such as Stephen King and Neil Gaiman, though Gaiman still produces great English) get away with this nonsense because their books sell.
I've not read many books by Anne Rice, so cannot say how her books changed, or how much the quality dropped over the years, though it is rumored the quality dropped significantly over time.
In Hamilton's case, she went from tightly plotted and fun little mysteries to plotless badly written porn quite abruptly in the course of one book. That book is also where the proofreading and editing stopped! But when people complained, it was because they were jealous haterz and stoopid proods, not because the books went from being good to awful in one fell swoop and because the author suddenly forgot basic spelling and rules of the English language. Blame the readers, not the author!
Cornwell is another author I stopped reading. The books started out as these great mysteries centered around a forensic scientist. They were rather gross, but well written. Gradually they changed so that they were filled with weird conspiracies centered around the main character and just got really...weird. Like having a werewolf guy in a story that was supposed to be firmly grounded in our real world. The characters changed from being flawed but likable into emo whiney people you wished would die so they would shut up. And she inexplicably changed from writing in first person to a very cold third person. And, like Hamilton, she also forgot to include things like a well integrated plot and basic rules of spelling and English usage. And like Hanilton, rather than taking any responsibility for the train wreck her series became, she looked elsewhere - in this case, the Pentagon!